Extruded magnesium alloy battery anodes



United States Patent Delaware No Drawing. Filed Feb. 15, 1960, Ser. No. 8,483

2 Claims. (Cl. 136-100) This invention relates to a process for improving the anode efficiency and shelf life of a primary cell utilizing magnesium as anode material and more particularly is concerned with a controlled process for extruding anodes of magnesium-base alloys for use in such cells.

Primary cells having magnesium as the active material of the anode, a cathode of finely divided carbon admixed with a depol-arizer of manganese dioxide, and an aque ous solution of an alkali metal, alkaline earth metal or ammonium bromide as electrolyte are disclosed in U.S. Patents 2,547,907 and 2,547,908. Further, it has been shown that addition of a substantially water-insoluble chromate of the group consisting of barium chromaite, lead chromate, and zinc chromate to the manganese dioxide-carbon cathode mixture of such cells gave increased capacity to the cells. (US. Patent 2,621,220). Also it has been disclosed in US. Patent 2,712,564 that reduced delayed action time, which time is defined as the time needed for a cell to obtain full working voltage upon reuse following a rest period after a use, as well as increased shelf storage li-fe prior to appearance of cracks in the anode of the cell are achieved if the magnesium alloy composition used in the anode was kept within certain limits. Advantageou-sly, \the anode material of the cell as described in US. 2,712,564 comprises a magnesium-base alloy containing from 0.1 to 0.7 percent of zinc, from 0.05 to 0.5 percent of calcium, the balance being commercial magnesium containing not over 0.005 percent of iron, not over 0.002 percent of nickel, and not over 0.1 percent of manganese. The alloy may also include up to about 1.5 percent of aluminum, the aluminum having the effect of giving the cell a higher capacity. The anode alloy may also contain Zirconium in amount between 0.001 percent and 0.1 percent. The higher amounts of zirconium are useable with the alloys containing little or no aluminum while the lower amounts of zirconium are useable in the alloys containing the higher amounts of aluminum. If desired, beryllium in amount between 0.0005 and 0.005 percent may also be included with advantage. Preferred proportions of the alloying elements as listed in this patent are: 0.75 to 1.2 percent of aluminum, 0.25 to 0.5 percent of zinc, 0.1 to 0.3 percent of calcium, the balance being commercial magnesium containing not more than 0.002 percent of iron, not more than 0.001 percent of nickel, and not more than 0.05 percent of manganese.

Primary cells prepared from the alloy composition disclosed in the "above listed patent exhibit low capacity and voltage along With a decreased shelf life (storageabil ity) when anodes of such cells exhibited the undesirable intergranular type corrosion. Inte-rgranular corrosion is that type of corrosion attack wherein there is pronounced, preferential progressive attack along grain boundaries whereby individual grains will be isolated and these then will undergo gross spelling from the metal proper. This type of corrosion is to be differentiated with what herein will be called general surface corrosion. In this latter type of corrosion attack, the attack is non-preferential and does not lead to such gross spelling of the individual 3,030,443 Patented Mar. 5, 1963 grains. A consequence of such intergranular attack upon the anode is reduced anode efficiency and reduction in shelf life. Now unexpectedly it has been found that if the magnesium alloy anodes are prepared by extruding alloys of substantially within the composition range disclosed in US. Patent 2,712,564 at a controlled extrusion rate and extrusion temperature, these anodes when used in primary cells undergo only general surface type corrosion upon extended storage of such cells.

It is a principal object of this invention to provide magnesium-base alloy anodes, which have substantially the same composition as disclosed in US. 2,712,564, for primary cells which do not undergo intergranular corrosion attack upon prolonged storage when used in such cells. It is a further object of the invention to provide a magnesium anode which exhibits a greater anode efliciency and a longer shelf life than has been obtained heretofore with this alloy composition.

Other objects and advantages will become apparent from the description of the invention and examples presented hereinafter in the specification.

In the practice of the present invention it has been found that magnesium primary cell anodes, substantially free of all intergranular type corrosion, are obtained by extruding magnesium base alloys containing up to 1.5 percent aluminum, from 0.1 to 0.7 percent zinc, 0.05 to 0.5 percent calcium, less than 0.005 percent iron and less than 0.002 percent nickel as well as from 0.01 up to about 0.2 percent manganese at an extrusion speed of from about 3 to about 10 feet per minute and a billet containerdie temperature range of from about 575 to about 625 F. Preferably these alloys will be extruded at an extrusion speed of about 5 feet per minute and containendie temperature of about 600 F. Magnesium-base alloys containing from about 0.75 to about 1.2 percent aluminum, from about 0.25 to about 0.5 percent zinc, from about 0.1 to about 0.3 percent calcium, the balance being commercial magnesium containing not over 0.002 percent of iron, not over 0.002 percent nickel and from about 0.08 to about 0.12 percent manganese desirably will be utilized for extrusion into anodes under these extrusion conditions.

In the preparation of such extruded materials, the extrusion ratio, which is the ratio of a billet container face area to the die opening area, will be controlled so that the anode stock is extruded within the rate and temperature limits given above. Theproper ratio to be used Will vary, as is Well known to one skilled in the art of metal extrusion, depending upon the desired final shape of the die-expressed (extruded) material. For example, with a flat strip extrusion of about .100 inch thickness and 2 inches Wide, as is used in the production of flat type cells, an extrusion ratio of about 33 to 1 will be used. Extruding a 2% inch diameter billet into a rod of about /2 inch diameter, which rod will be used as stock for preparing slugs for impact extrusion of cell cans (anodes) for N-sized cells, will be carried out under the conditions set forth by this invention at an extrusion ratio of about 36 to 1. On the other hand extrusion of about a 4 inch diameter by 1 foot long billet into a 1% inch diameter rod, used as stock for preparing slugs for the impact extrusion of D-sized cell cans, will be carried out at an extrusion ratio of about 9 to l.

The anode material is prepared by melting a suitable quantity of magnesium of a requisite purity preferably in a graphite or alloy steel crucible so as to avoid excessively contaminating the magnesium with other metallic elements especially iron, copper, and nickel. The metallic elements to be alloyed with the magnesium are usually introduced in this order; zirconium (if any), zinc, aluminum, beryllium (if any), and lastly calcium.

After melting together the metallic elements of an alloy, it is cast into billet form, scalped to remove the casting skin, and then die-expressed (extruded) under controlled temperature and speeds either into a rod of a suitable diameter for impact extrusion into cell cans (anodes) or into flat strips from which squares or rectangles are cut for use as anodes in the preparation of fiat cells. For example, molten alloy may be cast into a billet about 4 inches in diameter and about 1 foot long, and, after scalping, the billet can be extruded into rods 1% inches in diameter. Slugs for impact extrusion are made by cutting the so-extruded rod into pieces of suitable length. Slugs inch long are suitable for making cans for F-size dry cells (nominal diameter 1% inches, nominal height 3%.; inches) while D-sized cans, which have the same diameter but a height of 2% inches, require slightly shorter slugs. A suitable thickness for the wall of the can is 0.05 inch but other thicknesses may be used. Flat type anodes are obtained, for example, by extruding a 2 /8 inches diameter by 6 inches long billet into a strip of about 0.100 inch thickness and 2 inches wide. The anodes of desired length are cut directly from this 2-inch wide strip.

The following examples will serve further to illustrate this invention but are not meant to limit it thereto.

EXAMPLE 1 Anodes, about 2 inches square, were prepared from a 0.100 inch by 2 inch strip extrusion of magnesium-base alloy extrusion billet having a nominal composition of 0.9 percent aluminum, 0.4 percent zinc, 0.2 percent calcium, 0.1 percent manganese and the balance magnesium containing less than 0.002 percent iron and less than 0.002 percent nickel. The extrusion billet measured 2% inches diameter by 6 inches long and was cut from a direct continuous cast magnesium ingot. The extrusion billet was preheated to about 650 F., placed in a 3-inch diameter billet container and extruded on a 500 ton extrusion press through a 0.107 inch by 2.0 inch die at a container-die temperature of about 600 F. and at an extrusion rate of about feet per minute.

For control purposes, other anodes were prepared from similar strips extruded from billets of the same alloy composition but at the following extrusion rates and containerdie temperatures:

The effect of extrusion conditions upon the corrosion behavior exhibited by these anodes during shelf storage of primary cells prepared using these anodes was found to be reproduced by an accelerated 7-day stagnant immersion test. In this test, 2 inch x 2 inch coupons were cut from freshly extruded 0.100 inch by 2 inch strips. These coupons then were placed in 175 cc. of aqueous battery electrolyte solution (250 grams per liter magnesium bromide and 0.25 gram per liter sodium chromate) and kept inthis solution for seven days in a constant temperature room at a temperature of 21 C.i1 C. After this test period, the coupons were removed from the corrosion test medium. The corrosion product was removed by immersing each panel for about 1 minute in an aqueous percent chromic acid solution (by weight) containing a trace of silver nitrate, the solution being maintained at a temperature of about 200 F. The panels then were removed from the chromic acid, water-rinsed, dried and examined to determine the type of corrosion attack which each had undergone during the immersion test: The results of corrosion tendencies shown by the fiat anode stock extruded as described above are as follows:

Extrusion Container- Corrosion pattern Test N 0. conditions die temp..,

speed, it./ 13.

min.

5 600 General surface attack. 20 600 Intergranular-s urface attack.

5 800 Do. 20 800 Intergranular.

Anodes, 2" x 2" square, were prepared from the extruded strips for use in flat cell type 6 volt railroad lantern batteries. In making the cells using these fiat anodes, the electrolyte used consisted of an aqueous solution containing 250 grams per liter of magnesium bromide and 0.25 gram per liter of sodium chromate inhibitor. However, a water-soluble bromide of the other alkali metal, the alkaline earth metals or ammonium can be used. The cathode mix was composed of 89 parts of African manganese dioxide, 3 parts of barium chromate and 8 parts of acetylene black per parts of mix. This mix was moistened with the foregoing described electrolyte solution in the proportion of 320 cc. of electrolyte solution per 1000 grams of the dry mix. Each 2 inch by 2 inch square anode was wrapped with paper previously moistened with gelled or thickened electrolyte made by cooking a starch-dour mixture (75 percent starch, 25 percent wheat flour) in electrolyte solution in the proportion of one gram of the starch-flour mixture per 30 cc. of the electrolyte solution until the mixture thickened. The moistened cathode mix was die-expressed into a 2 inch wide strip of about four times the thickness of the fiat anode. The extruded cathode mix was cut into pieces about 2 inches long. A paper wrapped anode was placed in vertical position and centered in a 1 by 2%. inches by 3 inches high plastic bag and two pieces of the extruded cathode mix then were tamped into the bag on each side of the anode so as to make direct contact with the anode and substantially fill the bag. The weight of the wetted cathode mix for each cell was about grams. Four 71 inch diameter graphite rods, slightly longer than the anode, were then forced, one into each of the four corners of the bag, down into the cathode mix to form the electrodes for the cathode mix. The forcing of the electrodes into the cathode mix caused this mix to become tightly packed against the outer wall of the bag. Each of the four carbon rods was fitted with a brass cap at its top. A lead wire was aflixed to the anode and a second single lead wire connected together each of the four graphite cathode rods. Exploded mica then was poured over the mix and anode to approximately fill the bag, and this exploded mica in turn was covered with a. thermoplastic wax. A small hole was pushed through the Wax in order to permit venting of the cell. This hole was covered with tape while the cells were stored prior to discharge, testing or use. Three of these cells were then attached together, being connected in series, to form a fiat pack having an initial voltage of approximately 6 volts.

Cell packs were prepared containing pre-weighed and identified anodes from each of the four extruded strips described herein, and these packs were stored for six months prior to discharge. Following this storage period, the vent holes on the cell were uncovered and each pack was then discharged on a continuous basis through a resistance of 275 ohms until an end voltage of 3.6 volts was reached. The anode eificiency of each cell, which is defined as the fractional amount of anode consumed during cell discharge as compared to that which theoretically should have entered into reaction on the basis of electrochemical equivalence was determined as follows: The packs were removed from the continuous discharge after having reached the pre-selected 3.6 volt end-voltage. The

shelf storage is presented in Table II which follows:

T able II Type of corrosion on anode during Anode storage eifieienoy upon con- Test N0. tinuous discharge (average) 1 General Surface 55 2 Intergranular and General Surface (ten- 47 dency towards Intergranular) 3 Iutergranular 23 EXAMPLE 2 Using the same procedure for cell preparation and the same magnesium alloy anode composition as was used in Example 1, similar 3-cell, fiat type railroad lattern battery packs were made using 0.056 inch thick by 2% inch square anodes cut from strips which had been extruded at various temperatures and extrusion rates through a 0.070 inch by 2 /2 inch wire rectangular die orifice. These strips were extruded from a 2% inch diameter by 6 inches long billet at a 50 to 1 extrusion ratio on a 500 ton extrusion press.

The cathode mix used in these cells consisted of 91 parts African manganese dioxide, 3 parts barium chromate and 6 parts acetylene black per 100 parts of mix. This mix was moistened with electrolyte (250 grams per liter magnesium bromide and 0.25 gram per liter sodium chromate) in the proportion of 360 cc. of electrolyte per 1000 grams of the dry mix.

Cell packs prepared in this case were tested to determine their cell capacity using a modification of the standard test referred to as Railroad Lantern Battery Test on page 11 of the Circular of National Bureau of Standards C466," issued December 1, 1947. In applying this Railroad Lantern Battery Test of the aforesaid circular to these magnesium anode flat cell packs, the test was modified to the extent that the resistance used in the discharge was 10.7 instead of 8 ohms per cell and the test was concluded when the voltage of each cell in the pack declined to 1.2 volts instead of 0.9 volt as mentioned in the circular. This modification was made since only three of the magnesium anode primary cells give substantially the same initial voltage as do four of the zinc cells in such battery packs. In the Railroad Lantern Battery Test each pack of 3 cells was discharged through the resistances for the first /z hour of every consecutive hour for 8 hours each day (total of four hours of discharge per day) until the voltage of each pack declined to 3.6 volts. The packs then were removed from test and the anodes removed and treated as described in Example 1 to determine the efficiency of these in the various cell packs. Table III presents the cell capacities shown by this test as well as anode efiiciencies for anodes extruded under various extrusion conditions and container-die temperatures.

Table III Anode preparation Cell pack charac- (ertrusion conditeristies tions) Test Corrosion pattern No. on anode 1 Con- Anode Extrusion tainer- Capacedirate (feet/ die ity eieney min.) temp. (hours) (per- F.) cent) 1 .5 600 General Surface" 46. 5 52 2 20 600 Tendency towards 40. 0 44. 5

Iutergranular. 3 5 800 .do 43. 0 43 4 20 800 Intergranular. 34. 5 34. 5

1 Determined by accelerated stagnant immersion test using 250 g./l. MgBrz-I-(l25 g./l. NazCrO; as test medium.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that I limit myself only as defined in the appended claims.

I claim:

1. A method of preparing magnesium-base alloy anode stock for primary cells utilizing an electrolyte comprising an aqueous solution of an inorganic bromide selected from the group consisting of the water-soluble bromides of the alkali metals, alkaline earth metals and ammonium which comprises extruding said anode stock at a rate of from about 3 to about 10 feet per minute at a billet container-die temperature of from about 575 to about 625 F. from a magnesium base alloy billet containing up to 1.5 percent of aluminum, from 0.1 to 0.7 percent zinc, from 0.05 to 0.5 percent calcium, the balance being commercial magnesium containing not over 0.005 percent of iron, not over 0.002 percent nickel and from 0.01 to 0.2 percent manganese.

2. A method of preparing magnesium-base alloy anode stock for primary cells utilizing an electrolyte comprising an aqueous solution of an inorganic bromide selected from the group consisting of the water-soluble bromides of the alkali metals, alkaline earth metals and ammonium which comprises; extruding said anode stock at a rate of from about 5 feet per minute and at a billet container-die temperature of about 600 F. from a magnesium base alloy billet containing from 0.75 to 1.2 percent aluminum, from 0.25 to 0.5 percent zinc, from 0.1 to 0.3 percent calcium, the balance being commercial magnesium containing not over 0.002 percent of iron, not over 0.002 percent nickel, and from about 0.08 to about 0.12 percent manganese.

References Cited in the file of this patent UNITED STATES PATENTS 2,274,056 Geiger Feb. 24, 1942 2,659,129 Leontis et a1 Nov. 17, 1953 2,712,564 Fry et al July 5, 1955 2,787,692 Braeuninger et a1 Apr. 2, 1956 2,841,546 Robinson July 1, 1958 2,934,583 Stevens Apr. 26, 1960 

1. A METHOD OF PREPARING MAGNESIUM-BASE ALLOY ANODE STOCK FOR PRIMARY CELLS UTILIZING AN ELECTROLYTE COMPRISING AN AQUEOUS SOLUTION OF AN INORGANIC BROMIDE SELECTED FROM THE GROUP CONSISTING OF THE WATER-SOLUBLE BROMIDES OF THE ALKALI METALS, ALKALINE EARTH METALS AND AMMONIUM WHICH COMPRISES EXTRUDING SAID ANODE STOCK AT A RATE OF FROM ABOUT 3 TO ABOUT 10 FEET PER MINUTE AT A BILLET CONTAINER-DIE TEMPERATURE OF FROM ABOUT 575* TO ABOUT 625* F. FROM A MAGNESIUM BASE ALLOY BILLET CONTAINING UP TO 1.5 PERCENT OF ALUMINUM, FROM 0.1 TO 0.7 PERCENT ZINC, FROM 0.05 TO 0.5 PERCENT CALCIUM, THE BALANCE BEING COMMERCIAL MAGNESIUM CONTAINING NOT OVER 0.005 PERCENT OF IRON, NOT OVER 0.002 PERCENT NICKEL AND FROM 0.01 TO 0.2 PERCENT MANGANESE. 